Field of the Invention
The invention relates to a method for operating an exhaust-gas purification device in the exhaust of a combustion system, in particular in the exhaust of an internal-combustion engine of a motor vehicle, into the exhaust gas of which a liquid that is drawn from a reservoir through a supply line is introduced during the combustion, for chemical purification of the exhaust gas. Such a method is disclosed in Published European Patent Application 0 577 853 A1.
The invention also relates to a device for introducing a liquid into an exhaust-gas purification device, including a reservoir for the liquid and a supply line, connected thereto, for the exhaust-gas purification device.
The invention is also suitable for heating systems and other stationary combustion systems, provided they make provision for the purification of the exhaust gas in such a way that a liquid (such as, for example, urea solution) is introduced into the exhaust gas in sprayed or nebulized form or in another manner, as is provided, for example, in the catalytic purification of some industrial exhaust gases. In that connection, the introduction of the liquid may be a problem if, after a prolonged operating stoppage, the temperature has dropped so far that the liquid has an unduly high viscosity or is completely frozen. However, the invention is primarily intended for internal-combustion engines in motor vehicles.
In order to reduce the pollutants contained in the exhaust gas of an internal-combustion engine, in particular the nitrogen oxides, the principle of the regulated or controlled Diesel catalytic converter (CDC) has proved advantageous. That procedure is used primarily in internal-combustion engines which are operated with an excess of air, such as, for example, Diesel engines and lean-running engines. The procedure, which is based essentially on the method of selective catalytic reduction (SCR) is disclosed in numerous publications and patent applications, for example in German Published, Non-Prosecuted Patent Application DE 43 09 891 A1, corresponding to U.S. application Ser. No. 08/490,115, filed Jun. 12, 1995; German Published, Non-Prosecuted Patent Application DE 43 10 926 A1; and German Published, Non-Prosecuted Patent Application DE 43 15 278 A1, corresponding to U.S. application Ser. No. 08/551,791, filed Nov. 7, 1995. In the case of the SCR method, the nitrogen oxides, together with ammonia, are brought into contact with a selective catalyst and catalytically converted into environmentally safe nitrogen and water.
Due to the danger associated with the use of ammonia, namely the toxicity, and because of the nasal nuisance caused by ammonia, ammonia should not be carried in the motor vehicle in the case of an internal-combustion engine equipped with a CDC system. Instead of ammonia, an aqueous urea solution is carried in the motor vehicle as a reducing agent for the catalytic conversion of the nitrogen oxides. Ammonia is always produced from the aqueous urea solution by hydrolysis in the exact quantity instantaneously required to convert the nitrogen oxides.
It is an advantage of the substances, such as urea, for example, which are present in aqueous solutions and liberate ammonia, that the storing, manipulation, conveyability and meterability are particularly easy to solve technically. A serious disadvantage of those aqueous solutions is that there is a danger of freezing at certain temperatures as a function of the concentration of the dissolved substance.
The simple addition of an antifreeze founders because no antifreeze is known for such aqueous solutions, which effects an appreciable lowering of the freezing point in economical doses. Moreover, if an additional substance, such as an antifreeze, is used in that case, there is generally the danger that, in conjunction with the use of a reducing agent in the exhaust-gas purification system, undesirable byproducts are produced which are emitted along with the exhaust gas. For that reason, antifreezes cannot be used in such a method for exhaust-gas purification.
A further possibility for avoiding the freezing of the aqueous solution and for avoiding frost damage is in principle to heat those parts of the exhaust-gas purification system which carry reducing agent. However, in the case of mobile applications, especially utility motor vehicles (trucks), locomotives and ships (insofar as they are not provided with an internal energy supply), the electrical energy necessary for that purpose is not available, in particular during prolonged shutdown times. Thus, for example, a volume of about 100 l of aqueous urea solution would have to be protected against freezing in the case of a truck.